As mobile devices have been increasingly developed and the demand for such mobile devices has increased, the demand for secondary batteries has sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having high energy density and discharge voltage, into which much research has been carried out and which is now commercialized and widely used.
A secondary battery has attracted considerable attention as an energy source for power-driven devices, such as an electric bicycle (E-bike), an electric vehicle (EV), and a hybrid electric vehicle (HEV), as well as an energy source for mobile wireless electronic devices, such as a mobile phone, a digital camera, a personal digital assistant (PDA), and a laptop computer.
A small-sized battery pack, in which a battery cell is provided, is used for small-sized devices, such as a mobile phone and a digital camera. On the other hand, a middle or large-sized battery pack, in which a battery pack including two or more battery cells (hereinafter, occasionally referred to as a “multi-cell”) connected to each other in parallel and/or in series is provided, is used for middle or large-sized devices, such as a laptop computer and an electric vehicle.
A lithium secondary battery exhibits excellent electrical properties as previously described; however, the lithium secondary battery has low safety. For example, when abnormal operations, such as overcharge, overdischarge, exposure to high temperature, and an electrical short circuit, of the lithium secondary battery occur, decomposition of active materials and an electrolyte, which are components of the battery, is caused with the result that heat and gas are generated and high-temperature and high-pressure conditions caused by generation of the heat and the gas accelerate the above-mentioned decomposition. Eventually, a fire or explosion may occur.
Safety issues of the lithium secondary battery are even more serious in a middle or large-sized battery pack having a multi-cell structure. Since a plurality of battery cells is used in the multi-cell battery pack, abnormal operation of some of the battery cells may cause abnormal operation of the other battery cells with the result that a fire or explosion may occur, which may lead to a large-scale accident. For this reason, the middle or large-sized battery pack is provided with a safety system, such as a fuse, a bimetal, and a battery management system (BMS), to protect the battery cells from overcharge, overdischarge, and overcurrent.
Meanwhile, as the lithium secondary battery is cycled in use, i.e. as the lithium secondary battery is repeatedly charged and discharged, a generating element and electrical connection members are gradually degraded. For example, degradation of the generating element leads to decomposition of electrode materials and the electrolyte, by which gas is generated. As a result, the battery cell (a container or a pouch-shaped case) gradually swells. In a normal state of the lithium secondary battery, the safety system, such as the BMS, detects overdischarge, overcharge, or overcurrent of the battery pack to control/protect the battery pack. If the BMS does not operate in an abnormal state of the lithium secondary battery, however, a possibility of danger in the battery pack is increased and control of the battery pack for safety is difficult. Since the middle or large-sized battery pack is configured to have a structure in which a plurality of battery cells is mounted in a predetermined case in a fixed state, the respective battery cells, when expanded, may be further pressed in the restricted case. Under abnormal operating conditions, therefore, a possibility of a fire or explosion is greatly increased.
Particularly, in a case in which the battery pack is used in a vehicle, strong external force sufficient to cause physical deformation of the battery pack may be applied to the battery pack or a needle type object may easily penetrate the battery pack due to collision of the vehicle. In this case, a short circuit between the battery cells may occur.
Therefore, there is a high necessity for technology that is capable of preventing abrupt flow of electric current and a fire or explosion of the battery due to a short circuit and more efficiently securing safety of the battery.